Hi --- Legally he can keep your deposit if he can show that you changing your mind has put him out of pocket, that means the cost of the MOT at roughly £40 & the service £60 total £100 that you have paid, he could justifiably keep to recover his costs.
He would have had to make the car road legal anyway.... to sell it to anyone else, so the expenses he has incurred are not 100% attributable to yourself.
You can realistically say Goodbye to the £100 deposit you have paid though.
The further £300 cancellation charge he is not entitled to, he simply cannot add further conditions to an existing contract just because he feels aggrieved at the loss of the sale..... for which the £100 loss you have incurred should reimburse him fully. I think no matter how you explain the withdrawal from this purchase, he will still not be happy.....so be prepared to set the £100 loss against the savings made on the other vehicle, there is no further point in making what he will only want to see as excuses to him. !!!